Apparatus for guiding medical instruments during ultrasonographic imaging

ABSTRACT

A method and apparatus is disclosed, employed in combination with an ultrasonographic system, for assisting in guiding and placing at least one medical instrument into a target tissue during a minimally invasive medical procedure. The apparatus comprises: a reference means; a processing means in communication with the ultrasonographic system; and a mounting means for mounting the reference means in a predetermined relationship to an ultrasonographic transducer. The reference means includes a plurality of apertures arranged in an predefined manner which are sized to permit at least one medical instrument to pass therethrough. The processing means determines the spatial relationship between the target tissue and the reference means and further merges a representation of the plurality of apertures with the ultrasonographic image to form a positioning image. The positioning image assists in the guiding and placement of the at least one medical instrument into a target location by identifying a path to the target location via a selected aperture. Preferably, each aperture is provided with an internal adjustment means to permit minor adjustment to the selected path.

This application claim benefit to provisional application Ser. No.60/032,058 Nov. 29, 1996.

FIELD OF THE INVENTION

The present invention relates to percutaneous medical procedures andmore specifically, to a method and apparatus for facilitating theguidance of medical instruments when utilizing ultrasonographic or otherimaging techniques.

BACKGROUND OF THE INVENTION

One of the most important functions of clinical surgery is the resectionand removal of undesirable tissues. During conventional resectionsurgery, the practitioner targets the undesirable tissue and usingvisual and tactile control, manually resects and removes that tissue.Resection implies that an incision is made in the skin to visuallyexpose and gain access to the undesirable tissue. Not surprisingly,resection surgery usually results in considerable trauma to the patient.Until recently however, resection surgery was a generally preferredmethod of operating because the practitioner had confidence in theeffectiveness of the procedure. This preference was primarily due to thefact that resection surgery led to, in most cases, complete removal ofthe undesirable tissue from the patient.

Minimally invasive surgery is an alternative surgical technique in whichundesirable tissue is destroyed without necessitating resection andremoval of the undesirable tissue. Minimally invasive surgicalprocedures can be performed using one of several known surgicaltechniques, the selection of which is usually determined by the type andextent of tissue to be destroyed and the location of the tissue. Forexample, prostate carcinoma is a common type of cancer which may betreated by using a percutaneous cryosurgical technique (a hypothermiaapplication) in which the destruction of the tumour is accomplished byfreezing.

In modern cryosurgical procedures, at least one cryoprobe having thegeneral appearance and size of a conventional knitting needle isinserted into an undesirable tissue which is to be destroyed. Thecryoprobe is provided with cooling sites typically located at the tip ofthe probe and cryobalation is performed by employing one of a variety ofpossible cooling means. Examples of such cooling means include: boilingof refrigerants; cooling of refrigerants; Joule-Thomson effects, etc.During the cryosurgical procedure, only a few small punctures are madethrough the skin of the patient where the cryoprobes are inserted. As isapparent from the foregoing discussion, a main objective of minimallyinvasive surgery such as cryosurgery is to minimize surgical trauma.

Other known surgical procedures which may be performed in a minimallyinvasive manner include hyperthermia, biopsy, alcohol ablation, andradiation seed implantation, photodynamic therapy and brachytherapy.

Minimally invasive surgical procedures have been shown to be effectivefor percutaneous contrast media injection and aspiration biopsy. Typicaluses of this type of procedure include: ultrasonic PercutaneousTranshepatic Cholangiography (ultrasonic PTC) for suspected carcinoma ofthe bile ducts; ultrasonic Percutaneous Pancreatic Ductography(ultrasonic PPD) for suspected carcinoma of the pancreas; and ultrasonicPercutaneous Transhepatic Portography (ultrasonic PTP) for obstructinglesions of the portal vein. Other types of ultrasonographic aspirationbiopsy of small intra-abdominal masses are well known and their useshave been extended to biopsy of the thyroid and breast.

One of the primary disadvantages of performing these types of proceduresis that the practitioner is required to accurately guess the depth ofthe needle placement in the patient's body. Typically, the practitionerknows when the target tissue has been reached by the passage of a bodyfluid such as bile, pancreatic fluid or blood through the needle.However, the depth of penetration through the tissue is not known nor isthe position and size of the suspected lesion. As will be apparent, itis important that the tissue which is the focus of the biopsy iscarefully located such that the needle does not puncture othernon-target organs such as the gallbladder, aorta or spleen.Unfortunately, in many situations, the visual quality of two-dimensionalultrasonography does not represent these vital structures clearlyenough, especially when they are positioned behind the target tissue.

Guidance during minimally invasive procedures, and in particular,percutaneous procedures, refers to the ability to assist thepractitioner during planning of the insertion points, target locations,and instrument trajectory.

Using conventional minimally invasive surgical techniques andinstrumentation, imaging and guidance of instruments such as needles,probes and the like is very limited. Accordingly, the effective use ofsuch instruments for these types of procedures requires an unusuallyhigh degree of practitioner skill. Two-dimensional imaging of the tissueis limited by virtue of the fact that only a single plane is in view atany one time and the plane of the view may be at the wrong orientationto properly image the procedure. Further, the required orientation ofthe two-dimensional image to guide instruments from an insertion pointto a destination point may be impossible to achieve. This problemtypically results from the inability to place the ultrasonographictransducer at the proper location due to factors such as patientposition or bodily obstructions such as bone.

One of the most serious drawbacks of conventional techniques andinstrumentation is the limited success rate and complications which canoccur due to lack of practitioner skill and the significant level ofguess work which typically accompanies such procedures. This can beattributed primarily to the lack of adequate imaging and instrumentguidance indicated above. By way of example, in the past, when usingprocedures such as hypothermia, hyperthermia, and alcohol ablation, thepractitioner has had to estimate the placement of the instruments andmake an educated guess as to when to terminate treatment of the tissue.The educated guess approach when determining medical instrumentplacement, verification of placement and subsequently when to terminatetreatment, primarily arises due to the inability to properly image andguide instrument placement in the tissue. The consequences of such guesswork can be detrimental to the patient's health. For example, in thecase of the cryosurgical prostatectomy procedure, the prostate issituated in close proximity to the rectal sphincter muscle, colon,urethra and bladder. Over freezing of the tumour into these regions maycause irreversible damage to these proximal organs, resulting in, forexample, incontinence and impotence.

The above described disadvantages of minimally invasive surgicalprocedures were severe enough to make their use questionable for manyyears. However, recent improvement in two-dimensional andthree-dimensional ultrasonography technology has resulted in improvedresolution of the target image so that the extent of the affected tissueand efficacy of the treatment is more easily seen.

While the use of ultrasonography imaging has permitted visualizationduring a percutaneous medical procedure, the practitioner stillexperiences difficulties inserting and guiding medical instrumentspercutaneously to a selected target tissue. It is typically verydifficult to maintain a constant trajectory to a target tissue andaccordingly, during insertion, medical instrument often drift offcourse.

DE A 4010573 discloses a needle guide for carrying out ultrasound withan ultrasound probe. The needle guide comprises a flat plate of uniformthickness which has cylindrical openings provided in a grid pattern. Theopenings are provided for the guidance of puncture needles and thereforehave a diameter which corresponds to the diameter of the punctureneedles. The openings are conical in shape at the end which receives thepuncture needle. The needle guide also has a mounting portion providedto receive an ultrasound probe. However, this needle guide assembly isdeficient in that it does not permit precise control and positioning ofa medical instrument.

Accordingly, there has been a long standing need for an apparatus toassist in guiding medical instruments percutaneously to a target tissuewhich overcomes at least one of the above-described disadvantages ofconventional minimally invasive surgical procedures.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel apparatusfor assisting instrument guidance and placement during a minimallyinvasive surgical procedure which obviates or mitigates at least one ofthe disadvantages of the previously described methods.

According to one aspect of the present invention there is provided anapparatus, employed in combination with an ultrasonographic system, forfacilitating the placement of at least one medical instrument intotarget tissue, comprising:

a reference means including a plurality of apertures arranged in apredefined manner and sized to permit a medical instrument to passtherethrough, wherein each aperture is provided with an internaladjustment means which permits placement of the medical instrument aswell as simultaneously permitting movement of the medical instrument ina perpendicular plane to a face plane of the reference means;

a mounting means for mounting the reference means in a predeterminedrelationship to an ultrasonographic transducer;

a processing means for determining the spatial relationship between anultrasonographic image of the target tissue generated via theultrasonographic transducer and the reference means,

a processing means for determining the spatial relationship between anultrasonographic image of the target tissue generated via theultrasonographic transducer and the reference means,

wherein the processing means merges a representation of the plurality ofapertures in the reference means with the ultrasonographic image toassist in the placement of the at least one medical instrument in thetarget tissue via a selected aperture in the reference means

Preferably, in accordance with the apparatus of the present invention,the predefined manner of arranging the plurality of apertures forms aCartesian coordinate grid. Alternatively, the predefined manner ofarranging the plurality of apertures is a Polar coordinate grid.

Also preferably, in accordance with the apparatus of the presentinvention, the plurality of apertures are provided with an index markingscheme to assist in the identification of placement coordinates and theselected aperture.

Also preferably, in accordance with the apparatus of the presentinvention, the mounting means includes a transverse adjustment means foradjusting the reference means transversely relative to a long axispassing through the ultrasonographic transducer.

Preferably, each aperture in the reference means comprises an internaladjustment means comprises a ball disposed in a complementary socket,generally at a mid-portion of the aperture, the ball including a passagetherethrough sized to receive the at least one medical instrumenttherethrough and registerable with the aperture and, an axle orientedparallel to a front face of the reference means, in a plane passingthrough the passage, wherein the ball permits transverse adjustment ofthe medical instrument in a plane orthogonal to the front face.

In accordance with a second aspect, the present invention provides amethod employing an ultrasonographic system for facilitating theguidance and placement of at least one medical instrument in a targettissue, comprising the steps of:

i) positioning a reference means relative to a ultrasonographictransducer in a region proximal a site on a patient which facilitatesaccess to the target tissue;

ii) referencing the reference means to the ultrasonographic system todetermine the spatial relationship therebetween;

iii) obtaining an ultrasonographic image of the target tissue;

iv) via a processing means, generating a positioning image bysuperimposing an image of the reference means over the image;

v) from the positioning image, selecting a target location within thetarget tissue where the at least one medical instrument is to be placed;and

vi) from the positioning image, determining an insertion path to thetarget location and determining placement coordinates on the referencemeans.

The method may also include a step of placing the at least one medicalinstrument into the target tissue via the placement coordinates on thereference means, along the insertion path to the target location.

In accordance with another aspect, the present invention provides areference means for facilitating the placement of at least one medicalinstrument into a target tissue, the reference means comprising:

a reference means including a plurality of apertures arranged in apredefined manner and sized to permit a medical instrument to passtherethrough, wherein each aperture is provided with an internaladjustment means which permits placement of the medical instrument aswell as simultaneously permitting movement of the medical instrument ina perpendicular plane to a face plane of the reference means;

mounting means for mounting the guidance plate in a predeterminedrelationship to an ultrasonographic transducer; and

interface means to link the guidance plate with a processing meansoperable to determining the spatial relationship between anultrasonographic image of the target tissue generated by theultrasonographic transducer and the reference means.

BRIEF DESCRIPTION OF THE DRAWINGS

A presently preferred embodiment of the present invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows a perspective representation of a medical instrumentguidance apparatus in accordance with one embodiment of the presentinvention;

FIG. 2 shows a partial side elevation view section through a referenceplate in accordance with the embodiment of FIG. 1;

FIG. 3 is a partial perspective view of an elevation section viewthrough a mid-section of the reference plate;

FIG. 4 is a cut-away sectional plan view of an aperture disposed throughthe reference plate;

FIG. 5 is a cut-away perspective plan view of the aperture with amedical instrument inserted therein;

FIG. 6 is a cut-away perspective elevation view of the aperture with amedical instrument inserted therein and transversely adjusted; and

FIG. 7 is a partial side elevation view section through a portion of aninstrument guidance apparatus in accordance with the embodiment of FIG.1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An apparatus for facilitating the placement of a medical instrument intoa target tissue is accordance with one embodiment of the presentinvention is shown generally at 10 in FIG. 1. Apparatus 10 generallycomprises a reference means, which in the present embodiment is in theform of a reference plate 15, movably attached to an ultrasound assembly20 via a mounting means 25. A suitable ultrasound assembly 20 has beendescribed in detail in U.S. Pat. No. 5,454,371, the contents of whichare herein incorporated by reference, and accordingly, will not bedescribed herein.

As shown in FIGS. 2 and 3, reference plate 15 is provided with aplurality of regularly spaced apertures 30 in the form of a Cartesiangrid. The plurality of apertures 30 pass through reference plate 15orthogonal to a face plane 35. As shown in FIGS. 5 and 6, apertures 30are sized to allow at least one medical instruments, such as a biopsyneedle 40, to pass therethrough.

The Cartesian grid formed by apertures 30 may be provided with anindexing means (not shown) which facilitates the practitioner in theplacement of biopsy needle 40. Typically, the indexing means is in theform of alphanumeric markings on face plane 35 indicating the rows andcolumns of apertures forming the Cartesian grid. It is contemplated thatthe Cartesian grid of apertures 30 could be replaced with a Polarcoordinate grid structure and achieve similar results. In this case, theindexing means would be altered to indicate radius and degrees.

Mounting means 25 is preferably fixedly attached to a portion ofultrasound assembly 20 so as to establish a reference location betweenultrasound transducer 45 of assembly 20 and reference plate 15. Mountingmeans 25 preferably includes an adjustment means, such as hinge 38, suchthat face plane 35 can be adjusted transversely with respect to thelongitudinal axis of transducer 45. Reference plate 15 is shown in analternative position with respect to ultrasound transducer 45 in dashedline in FIG. 7. The adjustment of face plane 35 with respect totransducer 45 facilitates the projection of at least a portion of theCartesian grid of apertures 30 to be mapped onto the target location.Also preferably, reference plate 15 is moveable along the length oftransducer 45. This movement may be achieved by movement of thetransducer relative to a stationary plate and/or movement of the platerelative to a stationary transducer.

It is contemplated that a plurality of orientation sensors (not shown)may be included on mounting means 25. These sensors measure theorientation of face plane 35 in the X, Y, and Z planes with respect tothe longitudinal axis of transducer 45. The orientation sensors may beconnected to a computer to provide improved feedback to the practitionerduring the preplanning for the needle placement.

To further facilitate placement of biopsy needle 40, each of apertures30 is preferably provided with an internal adjustment means 50. As mostclearly shown in FIGS. 4, 5 and 6, internal adjustment means 50comprises a ball 55 disposed within a complementary socket 60. Socket 60is located at a mid-point along the length of aperture 30. Each ball 55has a passageway 65 therethrough sized to receive a portion of biopsyneedle 40. Passageway 40 forms a part of aperture 30. Each ball 55 isrotatable around an axle 70 which is parallel to face plane 35. Whenbiopsy needle 40 is received within passageway 65, rotation of ball 55around axle 70 permits movement of the needle in a plane perpendicularto face plane 35, as shown by Arrow A in FIG. 6.

As shown in FIG. 2, for ease of construction, reference plate 15 may beformed from two face-to-face plates 80 and 80′ held together by afastener such as bolt 90. The face-to-face construction facilitatesplacement of balls 55 in sockets 60.

In a presently preferred embodiment, apertures 30 are approximately 10mm long and have a diameter of about 4.75 mm. A 6 mm diameter ball 55 isplaced centrally in each aperture. Passageway 65 through each ball 55has a diameter of approximately 2 mm. With such an arrangement, theinternal adjustment means provides approximately a 30° range of movementfor a medical instrument (i.e., ±15° from the longitudinal axis of theaperture). If the diameter of the ball is increased to 8 mm and thediameter of the aperture is increased to 6.85 mm, then the internaladjustment means provides approximately a 50° range of movement (i.e.,±25° from the longitudinal axis of the aperture).

The operation of the reference plate will be described with reference tothe use of a three-dimensional ultrasound transducer, although, as willbe apparent, the reference plate is equally applicable for use withtwo-dimensional ultrasound. Once transducer 45 has been placed in thepatient, the apparatus is initialized so that the orientation of faceplane 35 and a specific reference aperture (not identified in theFigures) on Cartesian grid is referenced to transducer 45. Athree-dimensional image of the target tissue is then displayed on acomputer monitor and a digital image representation of the Cartesiancoordinate grid is superimposed over the three-dimensional image suchthat the Cartesian grid of apertures 30 corresponds spacially with theimaged region.

The practitioner then determines the desired target location for thebiopsy needle 40 and inputs this information to the user interface ofthe computer apparatus controlling ultrasound apparatus 20. Ultrasoundapparatus 20 then calculates the optimal trajectory of insertion inthree dimensions via an appropriate aperture 30 in reference plate 15.The results of this calculation are communicated to the practitionerthrough one of several possible means. Typically the results aredisplayed on a computer monitor by highlighting or changing the colourof the selected aperture on the positioning image or by display theappropriate index coordinates.

If the calculated trajectory of biopsy needle 40 to the target tissue isat an oblique angle to face plane 35, reference plate 15 may be moved,possibly under the guidance of the previously described orientationsensors, such that the orientation of face plane 35 is perpendicular tothe calculated trajectory path.

If more than one medical instrument is required for insertion into thetarget tissue, the foregoing steps may be consecutively repeated foreach instrument or, the plurality of target locations may be entered asa single step.

When the at least one biopsy needle 40 is inserted into the patientthrough the indicated aperture, the trajectory of the needle can bemonitored using real-time three-dimensional ultrasonographic imaging, asdescribed in U.S. Pat. No. 5,454,371, incorporated herein by reference.As will be apparent, the calculated trajectory of biopsy needle 40 canbe superimposed over the real-time image to further assist thepractitioner.

Internal adjustment means 50 provides the practitioner with the abilityto make minor adjustments to the trajectory of the instrument duringplacement. This is particularly useful if the instrument contacts animpenetrable mass such as bone or delicate tissue such as nerves orblood vessels. The minor adjustment also provides for slight movement ofthe target tissue within the body which ma occur as a result of thepatient's breathing. The limited movement of the internal adjustmentmeans within the chosen aperture helps ensure that the instrument doesnot deviate significantly from the preferred trajectory.

While three-dimensional ultrasonographic images are preferred forviewing placement of the instruments, other imaging modalities may beemployed with equal success. The three-dimensional ultrasonographicsystem presently employed is that disclosed in U.S. Pat. No. 5,454,371,the contents of which are herein incorporated by reference. However aswill be understood by those of skill in the art, other three-dimensionalultrasonographic systems may be employed with the apparatus and methodof the present invention with equal success.

In is further contemplated that three-dimensional reconstruction ofother imaging modalities such as Computer Tomography (CT) or MagneticResonance Imaging (MRI) can be used in addition to, or as a replacementfor, the three-dimensional ultrasonographic model. In this embodimentultrasonographic transducer 45 is manoeuvred until a real-timetwo-dimensional ultrasonographic image is produced of a plane showingcharacteristic landmarks of the organ anatomy or the target tissue. Thisplane is then found in a three-dimensional model generated using the CTor MRI modalities and an image is generated. When the two-dimensionalreal-time ultrasonographic image matches that of the three-dimensionalmodel image, the characteristic pixels are designated. These referencepixels can again be tracked on a computer to allow reorientation of thethree-dimensional MRI or CT model to the new organ position.

Although the present invention has been described with reference to theplacement of a biopsy needle, it is envisioned that the apparatus may bereadily adapted to may other medical instruments, such as cryosurgicalprobes, guidance sheathes, thermocouples an the like. Further, thepresent invention is not limited to use in cryosurgery. It is envisionedthat the instrument guidance techniques disclosed herein are equallyapplicable to other non-invasive surgical techniques such ashyperthermia, alcohol ablation, radiation seed implantation,photodynamic therap and brachytherapy.

The present invention has been described with reference to a presentlypreferred embodiment. Other variations and embodiments of the presentinvention may be apparent to those of ordinary skill in the art.Accordingly, the scope of protection sought for the present invention isonly limited as set out in the attached claims.

What is claimed is:
 1. An apparatus, employed in combination with anultrasonographic system, for facilitating the placement of at least onemedical instrument into target tissue, comprising: a reference meansincluding a plurality of apertures arranged in a predefined manner andsized to permit a medical instrument to pass therethrough, wherein eachaperture is provided with an internal adjustment means which permitsplacement of the medical instrument as well as simultaneously permittingmovement of the medical instrument in a perpendicular plane to a faceplane of the reference means; a mounting means for mounting thereference means in a predetermined relationship to an ultrasonographictransducer; a processing means for determining the spatial relationshipbetween an ultrasonographic image of the target tissue generated via theultrasonographic transducer and the reference means; wherein theprocessing means merges a representation of the plurality of aperturesin the reference means with the ultrasonographic image to assist in theplacement of the at least one medical instrument in the target tissuevia a selected aperture in the reference means.
 2. An apparatusaccording to claim 1, wherein the internal adjustment means comprises aball disposed in a complementary socket, the ball moveable within thesocket and including a passage therethrough sized to receive the atleast one medical instrument therethrough and registerable with theaperture.
 3. An apparatus according to claim 2, wherein the ball islocated generally at a mid portion of the aperture.
 4. An apparatusaccording to claim 3, wherein the ball is moveably mounted on an axleoriented parallel to a front face of the reference means, in a planepassing through the passage, wherein the ball permits movement of themedical instrument in a plane perpendicular to the front face of thereference means.
 5. An apparatus according to claim 1, wherein thepredefined manner of arranging the plurality of apertures forms aCartesian coordinate grid.
 6. An apparatus according to claim 1, whereinthe mounting means is attached between the ultrasonographic transducerand the reference means.
 7. An apparatus according to claim 1, whereinthe reference means comprises a generally rectangular plate.
 8. Anapparatus according to claim 7, wherein one face of the rectangularplate is shaped to fit the contours of a portion of a human body.
 9. Anapparatus according to claim 1, wherein the plurality of apertures areprovided with an index marking scheme to assist in the identification ofplacement coordinates and the selected aperture.
 10. An apparatusaccording to claim 1, wherein the mounting means includes a transverseadjustment means for adjusting the reference means transversely relativeto a long axis passing through the ultrasonographic transducer.
 11. Anapparatus according to claim 1, wherein the ultrasonographic system is athree-dimensional ultrasonographic system.
 12. A method employing anultrasonographic system for facilitating the guidance and placement ofat least one medical instrument in a target tissue, comprising the stepsof: i) positioning a reference means relative to a ultrasonographictransducer in a region proximal a site on a patient which facilitatesaccess to the target tissue; ii) referencing the reference means to theultrasonographic system to determine the spatial relationshiptherebetween; iii) obtaining an ultrasonographic image of the targettissue; iv) via a processing means, generating a positioning image bysuperimposing an image of the reference means over the ultrasonographicimage; v) from the positioning image, selecting a target location withinthe target tissue where the at least one medical instrument is to beplaced; and vi) from the positioning image, determining an insertionpath to the target location and determining placement coordinates on thereference means.
 13. The method according to claim 12, wherein theultrasonographic system is a three-dimensional ultrasonographic system.14. The method according to claim 13 wherein the method includes afurther step, concurrent with step viii), of monitoring placement of theat least one medical instrument along the insertion path to the targetlocation, via the placement coordinates, with one or more imagesgenerated by the three-dimensional ultrasonographic system.
 15. Themethod according to claim 12, further comprising a step of placing theat least one medical instrument into the target tissue via the placementcoordinates on the reference means, along the insertion path to thetarget location.
 16. The method according to claim 12 wherein, steps i)through vii) are repeated for a plurality of medical instruments.
 17. Areference means for facilitating the placement of at least one medicalinstrument into a target tissue, the reference means comprising: areference means including a plurality of apertures arranged in apredefined manner and sized to permit a medical instrument to passtherethrough, wherein each aperture is provided with an internaladjustment means which permits placement of the medical instrument aswell as simultaneously permitting movement of the medical instrument ina perpendicular plane to a face plane of the reference means; mountingmeans for mounting the guidance plate in a predetermined relationship toan ultrasonographic transducer; and interface means to link the guidanceplate with a processing means operable to determining the spatialrelationship between an ultrasonographic image of the target tissuegenerated by the ultrasonographic transducer and the reference means.18. A reference means according to claim 17, wherein the internaladjustment means comprises a ball disposed in a complementary socket,the ball moveable within the socket and including a passage therethroughand registerable with the aperture.
 19. A reference means according toclaim 18, wherein the ball is located generally at a mid portion of theaperture.
 20. A reference means according to claim 18, wherein the ballis moveably mounted on an axle oriented parallel to a front face of thereference means, in a plane passing through the passage, wherein theball permits movement of the medical instrument in a plane perpendicularto the front face of the reference means.
 21. A reference meansaccording to claim 17, wherein the predefined manner of arranging theplurality of apertures forms a Cartesian coordinate grid.
 22. Areference means according to claim 17, wherein the ultrasonographtransducer is a three-dimensional ultrasonographic transducer.